Power line carrier wave communication system



2 Sheets-*Sheet l R. C. EDSON ET AL POWER LINE CARRIER WAVE COMMUNICATINSYSTEM Filed March 9, 1946 R. c. 5050A/ NVEWORSJ W 5ML/NG ATTKORNEVSept. 13, 1949. R. c. EDsoN ET A1.

POWER LINE CARRIER WAVE COMMUNICATION SYSTEM Filed March 9, 1946 2Sheets-Sheet 2 ATTORNEY atented Sept. 13, 1949 NITED s #PATENT @ENCEPOWER LINE CARRIER WAVE COMMUNICA- 'rroN SYSTEM corporation of New Yorkv u Y pplication March 9, 1946, Serial No. 653-;254

8 Claims.

The invention relatesto an electric Wave transmission system, and.particularly to an electric wave communication system .employing highrequency carrier waves foratransmitting the .communication signals. .lt.

The invention isapplicable to acommunication system employing one.orrcnore high .frequency carrier channels to providetworway telephoneservice to telephonesubscribers .overa common transmission medium.I'n-,suoh asystem, the .common medium maybe a transmission line, forexample, one used primarily `forsupplying low .frequency, high voltage.electric-power to a number of power consumers. in a powerdistributionnetwork in a rural varea, or-,it-mayl comprise one or more wire andradio transmission links. .Each carrier channel may bentilizedforproviding twoway communication between telephone. subscribers associatedwith that-channel von a. party-line basis over the line or other commontransmission medium through an individualcarrier terminal having an-;associatedrncentral switching cnice which Amay be associated also withother carrier channels on the commonrnedium. Thercentral oihce mayprovide meansforfconnecting the carrier Atelephone subscribers-Astations associated with each carrier channel on the'commontransmission medium' in communication relation with each other or withotherwtelephone subscribers stations yassociated :with: other carrierchannels on the same medium orion-l outside systems.

The; inventionis specifically applicable to a carrier communicationsystem of the general type described lia'bove in whichthree difierenthigh frequency 4electridwaves per channel are employed as carriers ,forbothsignaling and speech transmission over theline orother commontransmission mediumisuch asis disclosed in the copending patentapplication of M` L. Almquist, Serial No. 625,083 filed October 27,194i. In such ya system,transmissionfrom the carrier terminal to thecarriertelephone subscriber associated with a `carrier channel on theline orrother common transmission medium is alwayson one of thesekfrequencies,.-say-thev frequency f1, and transmissionjromeach--subscriber,'s` station on that carrierchannel over the commonmedium to the eentraloice carrier ftermin-al is on eitherl one of` two-otherqcarrrer frequencies, say frequency f2 or f3,dependingonythertypeof call. (Voice frequency4 transmission `is providedbetween the central office and-the.carrierjterminal On central ofcecalls that i s -l Vcalls between-a carrier Isubscriber associatedvitlii.,oneac :arrierA channel on the common medium and, thefcentnalcnice, or

outside subscriber'connected tothat oice, transmissionirom .the .carrierterminal to the carrier subscriber .is on carrierifrequency f1 and fromthe carrier sulzvscri-berto,.the;4 carrier terminal. on .carrier.frequencyja .Dnreverting orV party-line calls, .thatis,fcallszbe'twe.er`1 twdcarrier subscribers on the same fcarriermhanneljon the common mediunnav calling..subscriber `transmits to the carrierterminalonetheacarrier.frequency f3, and receivesv from the;.-c'arriler.` .terminal lon carrier frequency f1, andthegcalledsubscriber transmits to. thev carriergterminal -oncarrier frequency f2and receives fnomthattermnal on carrier frequency-f1, meansbeing-provided at each subscribers station for. Shifting from thelnormal transmitting carrier frequency f2v to the alternate transmitting.Carrienfrequency f3 when the subscriber at .that station;.hasfinitateda. reverting call. In the systemiofnthe aforementioned patentapplication this frequency shiftingmeans is under control ofthe, callingsubscriber, a nonlockne key being providedfineaoh, canriersubscrbers setfor this pllpQS.

An object o the iinvention is to improve a carriersignalingsystemgof'the general y ,three-carrier frequency. typedescribedabove,particularly from the standpointdfofgautomatic operationen revert-Calls, y

Another object lis .to Supply two-way carrier telephbnc` ,serviceeciently and economically over, acominontransmission ,medium1 such as apower ,distribution line, ,toa ,number of telephone subscribers, forexample, to individual power consumers lserved byl--thesama power lineina rural area.A 1,. Y A

Other vobjects are to :provide two-way communictionbetweec any two, of.afnumbef ,0f Subscribers associated V-withfL the same5 carrier Achannel0h a.emmonfraasmisson mdium@ a Darty-.line basis.; teamlid?. two-nayCommunication between any subscriber associated withone carrier`channelforiy the common medium and another subscriber onanotherarrierchannel on the Samemedium-:Om an. Outs. e, Sysm @Ver the common mediumthrough-sacentral orifice; and to provideaconference connection betweenany two s ubscribersnonsthe same carrier channel on the common mediumand,any. out side connection (voice or other carrier channel on thecommon medium); thrQueh-the-entra1-Simca.

In.A accordancewith the present invention, three-frequencycarrieracomrnnnication system of the above-described generaltype 'isfmainlyftogpwvisaaitinerant@ trolmr ne and; Pirelli-0. ttmgolma level'mngCap-S- his carrier transmitter to be energized so as to send out acarrier of the normal frequency f2 over the common medium to the carrierterminal where it operates switching arrangements to bring in theoperator or dialing equipment in the associated central cnice, and toenergize the carg rier transmitter at that terminal so that it sends thecarrier frequency f1 over the common medium to operate busy signalingdevices at all subscriber stations. The calling subscriber then placeshis call with the central oflice by sending out his voice or dialingcall signals on his normal transmitted carrier f2 to the carrierterminal where they are demodulated to voice frequencies fortransmission to the central office. The calling subscriber then hangsup,and the carrier f1 modulated with coded low frequency (-cycle)ringing current is sent out from the carrier terminal under control ofthe central oce operator or machine switching equipment until the calledsubscriber answers and sends out his normal carrier frequency f2 toenergize the transmitter at the carrier terminal. The continuous carrierfrequency f1 then sent out over the line from the carrier terminaloperates switching equipment at the calling subscribers station to shiftthe tuning of the transmitter at the latter station from fz to fs. Whenringing ceases, the calling subscriber lifts his telephone set from theswitchhook to energize his transmitter which sends out the revertingcarrier frequency f3, since the frequency f1 initiated by the calledsubscriber was on the line prior to this operation of the callingsubscribers switchhook. The calling subscribers talking signals aretransmitted on carrier f3 to the carrier terminal where they aredemodulated to voice frequencies and remodulated on carrier f1 fortransmission over the line to the called subscriber. The calledsubscribers talking signals are transmitted on the carrier f2 to thecarrier terminal and are there demodulated to voice frequencies and areremodulated on carrier fi for transmission over the line to the callingsubscriber.

If a continuous carrier wave of the frequency f1 is not present on theline when a subscriber operates his hook switch, the operation of thelatter, in addition to energizing the transmitter at the station, alsodisables the switching equipment provided for shifting the transmittingcarrier frequency so that the normal carrier of frequency f2 is sentout.

These and other features of the system of the invention will be broughtout in the following detailed description when read in conjunction withthe accompanying drawing, in which:

Figs. 1 and 2, when placed side by side with Fig. 1 at the left, show inblock schematic form a power line carrier telephone system embodying theinvention.

Fig. 3 shows schematically one type of terminating network which may beused on the power line in the system of l1"'igs. 1 and 2 to adapt it for`carrier transmission; Y

Fig. 4 shows schematically one type of filter network which could beused for properly couayireis" pling the carrier terminal to the powerline in the system of Figs. 1 and 2, and for providing in addition asuitable termination on the carrier terminal end of the power line; and

Fig. 5 shows schematically one type of filter which could be used forcoupling each of the subscriber stations to the power line in the systemof the invention shown in Figs. 1 and 2.

The power line carrier telephone system in accordance with the inventionshown in Figs. 1 and 2 includes a transmission line I primarily employedfor transmission and distribution of low frequency, high voltageelectric power and adapted for simultaneous use for high frequencycarrier wave telephony. The line I is shown as a single phase lineconsisting of one phase conductor 2 and a neutral conductor 3, thelatter being grounded at a number of spaced points, for example, at eachpole. This type of line is coinmonly used in rural areas fortransmitting high voltage, low frequency (G0-cycle) electric power toelectric power consumers.

The portion of the power line I used for carrier transmission isisolated from the other portion of the line not so used by means of thecarn rier choke coils 4 connected in series with the phase wire 2 at thetwo ends of the carrier section, as indicated, these coils beingdesigned to offer a high series impedance to the carrier frequencies soas to minimize adverse transmission effects from the portion of thepower line not used for carrier transmission, but a low series impedanceto the low frequency (G0-cycle) power currents so as to enable thelatter currents to be transmitted freely over the portion of the lineused for carrier transmission to the power loads. Other suitableisolating or sectionalizing equipment may be employed in place of thechoke coils, for example, fllters or tuned circuits.

To prevent reflections which might make it difiicult to detect thecarrier frequency energy at various points in the line, it is desirableto terminate the ends of the portion of the line I employed for carriertransmission in approximately f characteristic impedance at the carrierfrequencies used; similarly, the ends of all b-ranches (not shown) ofthe power line used for carrier transmission should be terminated inapproximately characteristic impedance at the carrier frequencies used.The network 5 at the far end of the section of line I shown, for thispurpose may be, for example, of the type illustrated schematically inFig. 3. As shown in Fig. 3, it comprises a configuration of uncoupledinductances, condensers and a resistor forming a k-type half-section,band-pass filter network connected across the line, including acondenser 6 in one series arm, and a drainage impedance to ground whichmay be an inductance coil l, as shown, or a resistor, shunted by aprotective spark gap 8, connected in series with the condenser 6 betweenthe phase wire and the grounded neutral conductor. The condenser 6, ofvalue in the order of 2,000 micromicrofarads, serves as a high impedanceto the high voltage (7.2 kilovolts), low frequency (60 cycles) powercurrents transmitted over the line I, and the inductance 'l of value inthe order of 10 millihenries, has a, relatively high impedance over thecarrier frequency band.

A plurality of like subscriber two-way carrier telephone stations A, B,C associated with one carrier channel on the power line I, that is, alltransmitting on the same carrier frequencies and receiving on the samecarrier frequency, are bridged across that line at different points byaje'sieis" tile @mm1-er1 ltirangtn-efiis??,ifaria sa individualtwoiway"carrier'terrnna statlon C'I formatearpower line I' esseri e,o'f-yaiue' in the pid'r-pr 1,000" to 2'oco-l i micromicrofaradsf,7 inseriesf witlfi "an inductance coil l shunted by"a"`prote tivf sparkgapn,` clonnected irf'shunt' with the phasenwirerand neutral wire of theepowerline. An 'impedance transformer, 'band-pass filter networkilconnects the subscribers' carrier telephonet set 'Y across "j the`inductance 'l'.4v The `network` I Ifis designed to'selectively transmit'a narrowba l"of frequencies, for example; a band 'extending mittingcarrier' frequency is "185 'kilocycles vand the receiving carrierfreduencyfisfl65 kilocycles, in each Asubs'cribers`carrie telephone setas in the' system to. bedescribede;Av andfals'o'v to vprevent band andthuswiu cause'biuy' asmaubridging loss on line" I. Inad'riiitioir'lf'owing tothe nature of a mid-seriesterminated"baiidpasslt'er such as shown, the Aimpedance ill beyry'hig'h (and largely"reactive)` outside t e pa's'sband and' thus channels Aexcept the oneftowhich; the 'associated subscriber set is assigned. If subscriber stationis at the end' ofthe seculieri of line" I, Vor branch thereof, used for`cartrier;'trarisriiission, the'nein worl: ofthe type illustrated'n'figfin'place of the terminating resistor element"ofthe latter.

i fromieo'tc 20oy kilocycles, if' the normal' transmitting'- Acarrierfrequency is 195 kilooycl`es the `reverting trans'-V The terminalcouple'rarrangement ID also inay @rated in Fig. 4, wnichdifrers'essentiauyfmm the subscriber'coupler" arrangement'of Eigj iin` that itsimpedance transformer; bandLpa'ss' filter' network I2 isdesignedtoselectively` transmita` wider frequency band (15()y to 485kilocycles) and to present an impedance on th line side l suitable forterminating the"'carrie'r terrninal end ofthe portion yof power line' VIe'IiipIcSyed' for carrier transmission in approximatelycharacteristicim- `pedance at' the carrier" frequenciesus'edjso as to preventreiiectionstendi'n'g to' jnali di'fticlt'the detection of theoarr'ierffrequency energy in carrier terminal CT. 'lhl'lsfitsfv 'use'inak'es"unneces-Y receiving'circuit" I 6` *i I 5 includes in order,

bin'dinca standard?lhafdtelepiione 'set i8' 'with the telephonereceiver' Itifavrriiulator"aiidassofA ciated carrier oscillatorforsupplying tothe modL ulator the carrier oscillations of the normaltransmitting frequency f2 (195 kilocycles) or the re. vertingtransmitting car'rier4 frequency fa. ,(185 kilocycles), represented' bythej` box: 20. and the associated piezoelectric crystals YI and Y2 forYrespectively tuning the carrie'rOscillator to the.v normal carrierfrequency .f2 195 kilocycles) and' the reverting, carrier frequencyfa`.(1851kilocyfcies); the high frequency amplifier 21'; the transinittingband-pass filter 22; and the'plugl-,in resistance equalizer pad 23feeding into'the line 'I through the coupler device 9.A The high fre;quency receiving circuit I5 includes in order, reading from left toright, the plug-in resistance equalizer pad 2li fed from the line 'I'througlrithe couplerdevice 9; the receiving'lbandlpas's filter.. 25; thehigh frequency amplierZS whichfrnay compriseY one or more vacuum'tubestages with.l interstage and output ltering" networks fortunf ing it toamplify properly the carrier signalsoverv the frequency rangetransmitted by the preceding band filter, and an automaticvolufnecontrol'y circuit fi? associated with the amplier, which may be of anyof the well 'known types, utilized for controlling the'gain 'of theamplifier to c0111-, v pensate for variations in the amplitude level ofthe received carrier signals due to variations in the less of thepreceding section of line; the demcdulator 2B; and the telephonereceiver I9 of the standard hand telephone set IBLv The modulatorincluded within'the boX'Z in the transmitting circuit I5 may be of anysuitwm cause 'negligibletridgihglessfof al1 carrier :i able unbalancedtype, includ-ieg 0116. 01 more electron discharge tubes or othervariableresistance devices, adapted for combining high fre. quencycarrier oscillations supplied to its carrier input terminals with audiofrequency signals ap.-`

Work `H my'b-con-et-dfda termatihg nef/ 9;. plied to its signal inputterminals, so as to .pro-

duce signal side-band components as well as the unmodulated carriercomponent in its output. The carrier oscillator in loo-X 20 may also beof any suitable type, and may be .combined with the associated modulatorin a single electron discharge tube in conventional manner. Thedemodulator 25 in receiving circuit I5' may be of any suitable type, forexample, itmay comprise a varistor consisting' of a plurality of copperoxide or crystal rectifier elements connected in series between theoutput cf amplifier 23 and the telephone receiver I9. The transmittingband-pass filter 22 in the transmitting circuit I5 is de-V signed toselectively transmit the voice ksignalmodulated carrier waves (one ormore signal sidebands) of the high frequenciesjz (195 kilocycles)A andfg (185 kilocycles) which may ,be ,applied` thereto from the output ofthe oscillator-modulator 2n, as well as the unmodulated. carrier` frequency components of these frequenciesand the band-pass filter 25 inthe receiving circuitll isL designed to selectively transmit the voiceYsignalmodulated carrier wave of the hi'ghffrequency; f1 (165kilocycles), aswell astheunmodulated carrier of that frequency, receivedfrom the line I.

Each of the subscribers statiOnsA, B, C l also includes thetelephonel'iook'switoh 29 Iassociated with the handset i8`; a suitablepower sup-v ply 3i, such as a batteryor'rectified'ISO-cycle"alfternating current power source, ia'daptdfor coril nection throughswitching contacts'onthe hooliff switch '29, V'when thehai'ids'tI'ernvo'vdffrom'f` the hook, to the modulatoros'cifllatforA Ziilandlt'hehigh* vfrequernzy amplieri 12 l "in "the transmitting .u

vcircuit it; a conventioaal'caiung 'uia'r 32" con!" www.

nected :in thek power supply of the modulatoroscillator and a normallyclosed monitor key 33 which Vmay be opened by the subscriber to openthe` power supply circuiti (from battery 3|) to disable transmittingcircuit |5 when he desires only to monitor (listen-in) on thetransmission over the power line I'. Each of these stations alsoincludes the electromagnetic switching relays R| and R2 controlled fromthe output of the demodulator 28 in receiving circuit, IG, and stationsignaling apparatus including a voice frequency ringer 34 and a busysignal lamp 35 controlled from theserelays in a manner which will bedescribed in detail later in connection with a description of theoperationfof the complete system.

The carrier terminal station CT comprises a transmitting circuit 3G andtwo receiving circuits 3`| and 38. The ouput of the transmitting circuit36 and the inputs of the two receiving circuits 3'l and 38 are connectedin parallel with each other and through the common coupler device I0 tothe'power line I. The input of the transmitting circuit 36 and theoutputs ofthe two receiving circuits 3l and 38 are ladapted forconnection by means of the hybrid coil 39 and associated normal linebalancing network 45 to the two-wire voice frequency line 4| leading tothe central telephone switching oiiice CO for the subscriber stations onthe power line. The central office CO, which may be of the manual orautomatic type, may be utilized also for connecting the telephonesubscriber stations on the power line to any of the telephone subscriberstations outside the carrier system in an associated telephone network.The central office CO is normally disconnected from the carrier terminalstation CT due to the normal break in the line 4| provided throughparallel connected, normally open switching contacts on the "controlswitching relays R3 and R4, respectively. The operation of either ofthese relays to close its contacts will enable the line 4| in its outputso that the central cnice CO will be connected in transmission relationwith the carrier terminal CT through the hybrid coil 39. An alternateline balancing network 44 is adapted to be connected to the networkterminal of the hybrid coil 39 in place of the normal balancing network43 by operation of the control relay R4 I when the carrier terminal CTis receiving carrier oscillations on the reverting frequency f3 (185kilocycles) in the manner which will be described later in connectionwith the description of operation of the whole system.

The alternate balancing network 44 may comprise a single terminatingresistor or other simple network such as to provide suitable unbalanceof the hybrid coil 39 so that voice signals may be transmitted throughthe latter from the receiving circuit 38 to the transmitting circuit 36on a reverting call, with as little loss and distortion as possible, andto provide substantially the same degree of loss for all frequencies inthe voice frequency band. The normal balancing network 43, on the otherhand, should comprise a more elaborate network suitable for providingadequate balance over the voice frequency range of any one of thedifferent impedance lines which may be' connected to the terminal by theassociated central office CO, and ifi-particular is designed to providethe best possible balance on long distance central office calls.

The transmitting circuit`36 of the carrier termlnal CT includes in orderbetween the hybrid coil 39 and the -vcoupler devicev |07, reading fromleft to right, a `voice frequency amplifierY andra.

modulator with an associated oscillator for supplying thereto carriervoscillations of the frequency f1 165 kilocycles), represented by the box45; and an automatic voice frequency volume limiter 46 which may be ofany of the well-known types, associated with the voice frequencyamplifier for controlling the gain of the latter so as to limit thevolume level of the Voice frequency signals applied to the modulator tothat value which will provide adequate modulation on long distanceincoming ca llsf4 from outside subscribers while preventingovjerloadingof the modulator on incoming calls from nearby outside subscribers orsubscriber stations on the power line; a high frequency amplifier 41 foramplifying the Waves in the output of the modulator; and thetransmitting band-pass filter 48 designed to selectively transmit one orboth signal side-band products as well as the unmodulated carriercomponent of the frequency f1 (165 kilocycles) produced in the output ofthe modulator. The voice frequency amplifier,l modulator and carrieroscillator included in the box 45 may comprise sept arate elements ormay be combined in a single multigrid electron discharge tube as iswellknown in the art.

The receiving circuit 31 in the carrier terminal CT includes in orderbetween the coupler device l0 and the hybrid coil 39, reading from rightto left, the receiving band-pass lter 49 adapted for selectivelytransmitting the speech-modulated and unmodulated carrier waves of thenormal frequency ,fz kilocycles) received over the line from thesubscriber carrier stations; the high frequency amplifier 55, which maycomprise one or more Vacuum tube amplifying stages with associatedinterstage and output band-pass iilters for tuning it over the frequencyband transmitted by the. band-pass iilter 49; an automatic Volumecontrol circuit 5| of any of the wellknown types associated with thisamplifier for controlling its gain so asvto compensate for variations inthe amplitude levels of the applied carrier signals due to lossvariations on the power line I; and the demodulator 52 for demodulatingthe applied`high frequency modulated carrier waves to voice frequencies,which demodulator may be of any of the well-known types, such as onecomprising copper-oxide or crystal rectiers.

The receiving circuit 38 includes in order between the coupler device l0and the hybrid coil 39, reading from right to left, the receiving band-,Y pass lter 53 adapted for selectively transmitting the speechmodulated and unmodulated carrier waves of the reverting frequency f3(185 kilocycles) which maybe received over the line from the subscribercarrier stations; a high frequency amplifier 54, which may comprise oneor more vacuum tube stages tuned to amplify elnciently the band offrequencies transmitted by the band-pass filter 53; an automatic volumecontrol 55, similar to the automatic Volume control circuit 5|,associated with amplifier 54; and the demodulator 55, similar to thedemodulator 52 in the receiving circuit 31, for demodulating the appliedhigh frequency signal waves to voice frequenc1es.

The output of the Vreceiving circuit 38 is normally disabled due to thenormal break in the circuit connections between the demodulator 56 andthe hybrid coil 39 provided by one set of normally openswitchingcontacts of the unoperated relay R4. The switching relay R3 isconthat station, the operation of. the relay RI by the receivedcontinuous carrier of .frequency f1 will cause the associated controlrelay R2 to be operated from battery 3|,v and then locked operated fromthat battery through theoperated righthand front contacts. of. thelatter-relay. The closure of the leftlhandcontacts of operated relay R2will cause the .busy signal lamp 35 at the station to be lighted frombattery 62. Thus, all other subscriber stationsB, C.

In a dial area,y the .dialtone automatically -transmitted over the voicefrequency line lll from the central office CO to the carrierterminal CTin response to the closure of the voice frequency line 4I atthatterminal byrelay R3 controlled by the rectified carrierof .frequencyf2 from the calling station A,.is applied by hybrid coil 39 to the inputof the oscillator-modulator to modulate the outgoing carrier offrequency Vf1 (165 kilocycles) supplied to lineI. .In -a manual area,speech fromthe. operatorfat .the central office CO is transmittedoverf-ftheyoice frequency line liI and applied .through-hybrid coil 39to the oscillator-modulator .45` -to modulate the outgoing carrier of.frequencyjr .Atthe calling station A, the dial tone or operatorfsspeech signalsY modulated on the carrier .f1-will pass through thecoupler device 5lv andreceiving-circuit I6 at the calling subscribersstation (station A) to the demodulator 28 in :which they'will. bedemodulated to voice frequencies and-will be Vheard by the callingsubscriberin histelephone receiver I9.

The subscriber atthecalling station A then talks into his transmitter.to tellthercentral oice operator to which outsidesubscribers station hewishes to be connected, or `operates his calling dial 32 in the usualmanner to dial the number of the station being called.` In theformercase, As signals will be combined `inhis-modulatoroscillator 26 with thecarrier oscillations of the normal frequency f2 (195 kilocycles) and themodulated carrier will be transmitted out over thelinel to thedemodulator 52 inthe receivingcircuit 31 0f the 4carrier terminalstation. CT.-as. previously described for the unmodulated.carrier.A Thedemodulated voice signalsofA. in `the output of the demodulator 52 willpass through the hybrid coil 39 and voice frequency lined! (maintainedclosed by the continued Voperationof -relay.R3) to the operator in thecentral oilce CO.-

When the subscriber at the. calling station A dials the number to becalled on his dialing device 32, this operation .causes the. transmittedcarrier of frequency f2 (195 kilocycles) to be interrupted inaccordance` with the digits dialed. The control relay R3 associated withthe demodulator 52 in the receiving .-circuit 31 at the carrier terminalCT follows thereceived 195 kilocycle pulses, the corresponding closuresand releases of its left-hand contacts; operating to relay the dialingpulses onto. the central Yofce over the voice frequency line 4I wherethey direct the switches (not shown)..in.the.selection of the calledparty.

The` central oilce operator or themachine switching equipmentinjthe.central oiiice'calls the called subscriber. on the -associated networkin the usual manner, and sets up a talking circuit between the calledsubscriber. and Vcalling subscriber A on the powerf line throughthecentral office CO, so thattheycan-converse with each other. f. i

The voice-signalsfof the :called outside subscriber Will be transmittedat voice .frequencies froml the central oiiiceCO'vov'er'the Vvoicefrequency line 4I to the carrier terminal station CT CPI are informedl:- Ythat the line I is being.. utilized on a call.

and will be impressed by the hybrid coil 39 on the transmitting circuit36 at that terminal. In

- that circuit, the called subscribers voice signals will be ampliedlandthen modulated in the device 45, which is maintained energized by thecontinued operation of relay R6, on the carrier oscillations of thefrequency f1 (165 kilocycles), and 'the modulated carrier wave will beamplified in Vthe high frequencyramplier 41 and passed out through thetransmittingband filter 48'and the coupler device II) to the power lineover which it Will be transmitted tothe calling subscribers station AA.At the latter station,'the voice-modulated carrier wave of frequency f1(165 kilocycles) will pass through the coupler device() and the plug-inresistance pad 24 and will be selected by the receiving band filter 25in the receiving circuit I6 at that station. The selected signalsl ofthe outside subscriber .will be amplified in amplifier 26, demodulatedinthe demodulator 28 and will be heard by the subscriber A in `histelephone receiver I9. Y. f

The voice signals of the calling subscriber at station A will betransmitted from his station as modulations of his normal carrierfrequency fz kilocycles) over the rline I to the demodulator 52 in thereceiving circuit 31 of the carrier terminal CT inthe manner previouslydescribed for the unmodulated carrier. of that frequency. As voicesignals in the output of the demodulator will be divided by the hybridcoil 39 between the voice frequency line 4I and the transmitting circuit36 of the carrier terminal. The portion entering voice frequency line 4|will pass over that line through the central roce CO to the calledsubscribers line. The other portion of As voice signals will bemodulated in the transmitting circuit I6 on the transmitted carrier Waveof the frequency f1 (165 kilocycles), and the resulting voice modulatedcarrier will be returned over the power line I. to the callingsubscriber's station where it will be selected, amplied, demodulated andheard in its receiver I 9 as side tone. f

At the end of the'call, the subscriber at station A will hang hishandset I8 up on hook switch 29 `causing the opening of switchingcontacts in the latter to disconnect the power supply 3| from themodulator-oscillator 26 and high frequency amplier 2 I. This will stopthe transmission of the carrier frequency f2 (195 kilocycles) over theline I. At the carrier terminal CT, the cessation of the receivedcarrier f2 will cause the relay R3 to release to open the voicefrequency line closure at the terminal, and the subsequent release ofthe relay R6 will remove the energizing power supply from the apparatusin the transmitting circuit 36 to stop the transmission of the outgoingcarrier frequency f1 and will return the other circuits in the carrierterminal to normal.

Operation on a central office call originated through the central o'ceOn a central office call originated through the central office CO to oneof the subscriber stations A, B, C on the power line I, the operator ormachine switching equipment in that oice applies coded ringing current,say of 20 cycles per second, between one side of the voice frequencyline 4I and ground. At the carrier terminal station CT, this current isrectified in the rectifier 51 causing the operation of the associatedrelay R5, while the supply of ringing current continues, to

carrelages :connect an-.-.energizing. circuit from battery'ill'fto.thenvoice :frequency amplifier yoscillator-modulator w15 and'.the`1high frequency amplier' 4l in the `transmittingcircuit :3.5 atthe terminal condition- I.ing that transmitting circuit to send outamplied carrier oscillations .of the frequency ,f1 .(165 kilocycles)through ithe :coupler device Iii to the line; and also to connect thevoice irequencyline BI to f the'finputo'f the oscillator-modulator 45lsothat `the oodedjZO-'cycle ringing signals from .the cen- 10 tral nfcewill fbesupplied to the modulator to "modulate, the carrier of'frequency f1' (165 kilocycles) transmitted out` over the line I` duringthe'duration of the applied ringing. tone. The

-carrier'sofzfrequency f1 '(165 kilocycles) modu- 15 "latedWith-.theZO-cycle ringing signals is diverted through the couplerdevices 9 into the receiving circuit I6 'of'. all subscribers stationsvA, B, Cv on the power line I. In the receiving circuit'vl at veachsubscribers station this ringing signal- 20 ymodulated carrierfr (165Ikilocycles) will be passed through the pad 24 and the receiving bandiilter "25, amplied iin amplier 26 and demodulated'in'demolulator 428causing the operation of the associated -control r'elay- RI at the20-cyc1e 25 "ratawhichinturn will lcause the voice frequency yringer 34at "theistation to be operated at the 20- cyclezcoded rate. AA .circuitfor operating' the ringerJ 34 isclosed by charging the condenser 63 inseries with the ringer 34 when relay RI is oper- 30 ated. The condenser63` is discharged through the ringe1t34`during the periods in which therelayRI is released.

Thesubscriber at the called station on the pow- Iremovingthe'handsetI'from the hook switchV 29 at hisv station. f' ThisA will `cause themodulatoroscillator`2c and :highV frequency amplifier 2l in his'transmitting circuit `I5 to be energized from battery 3ithrouglroperated switching contacts 40 onY the'iho'ok switch 2Q, andtheoperating circuit .for the'control relayRZ from battery 3i to be openedbythe opening of other associated switching .contacts on the hook switch25. If the relay therelay through its locking contact, so as to connectlthe 185-kilocycle crystal Y2 in the ire- 'quency-determining#circuit ofthe modulatoroscillator 20Tin placeV of 'the normal 195-kilocyclecrystalYI l, the yreverting carrier frequency fa (185 -kilocy'clesl maybe-1nomentarily applied to the poweriline through the transmittingcircuit I5 and coupler device 9. This Vmomentary spurtof carrier`frequency f3f (185 kilocycles) will have no eiectV on the switchingcircuits at the carrier ter- `155 rminal CT` inasmuch as theV switchingrelay R4 associated with the demodulator 5E in the receiving circuit" 3Bof' that terminal which selects that frequency,cannot beoperated due tothe break in its energizing circuit through the normallyfm open`switching contacts on the control relay R3. i The 4latter-willunot be'operated in the absence of 'the supply ofthe normal carrier frequency f2(195 line I from the-output of the oscilltaor-modulator 2tin`transmitting circuit i5 of the called subscribers` station.

At the carrier terminalCT, Vthis carrier wave of frequency f2 ('195ykilocycles) will be diverted' from 75 `er `line;recognizinglhis codedring, answers by 35 v R2 lis'oper'ateclon-'the ringing current suppliedto 45 thexlinea throughf. the `*coupler device Iainto :the

carriert'erminal' CTand. will'be selected' bythe -ilter 119 sin; thereceiving circuit 31 therein. The

-selected carrier fziwillzbe ampliiedin'amplier 50 and rectieddemodulator 52 causing the oper- Vationcoftherassociated relay R3lin the-manner vlpreviously;describedttov provide a: closure. of the.voiceifr'equency line Alito the central oiceLCO, f andtozoperate thecontrol relay RB. The operation vof Ithe relayl R6. causes:- vthe`modulator-oscil- .Llator Iarid -1 voice i frequency amplifier lapparatus (box: 45) indthe transmitting. circuit 351at the, teraminaltocbe. energized from batteryl 6I sothata `scriberslstationsfA, B, C,.eXcepttheHcalled subsciiberls.` station, ,the received carrierlofrfre- -quency f1 .willfoperate the relay R2.. which in turn :Willcausethe busy .lampfSL-atthe station :to be lighted.

...The closure of the voice frequency line-Mat the carrier.'terminaljCTin responseJ to. the operation :ofthe-control'relayRS. bythe. rectied carrier of the; normal. frequency .f2 (195 .kilocycles)will trip the ringingfromthe. centraloiiice. The outside subscriber andthe called carrier-subscriber then can communicate with each other overthe talkingpaths-which are set yup through the 4central cnice CO.

1When` thecall is completed, and the calledV subscriber-'on thelpower'linehangs up his handsetfon the-hook; his carrier transmittingvapparatus isdeenergized? `byfthel openingof certain switchingcontactson the hook switch 29 vbreaking the energizing 'circuitf frombattery 3 i and transmission yof the-*normal carrier frequency f2 (195kilocycles) -overfthe'power lineA I ceases. This causes the re- 'layfRtobe'releasedat the carrier terminal CT. "Ihe release ofthe-relay R3 inlturn causes the release-of the relay'RS, and `the resultant opening ofthe voice frequency line 4I to the central olce CO transmitsa disconnectsignal to that oui-ce.

The relay R51' at the carrier terminal CT isa slow-release relay. Duringthe releasing time of this relay, the relay'PlZ in the calledsubscribers stationwill vmomentarily operate to light the busylampiathis-station. 'With the release of the relay R6 at the carrierterminal CT, the apparatus `in thetransmitting circuit at that terminalis delenergized so that the carrier f1 (165kilocycles) Yis removed fromvthefline I. "Therefore, the operated ARI vand R2 relays in all of thesubscribers stations' A, B, C will Irelease and the busy signal lamps'at these stations-will be extinguished.

Ifa subscriberat'one'of the stations A,B, `C on'the power line. whenconnected through Vacomrnon battery manual central oice should ash hisswitchhook. the* circuit operation will be asiollowsr On the rsthang-upof his handset I8, thecarrier transmitting apparatus at hisstation :will 'bedeenergized This will cause the relay R3 atvthe'carriierV terminal CT tobe released vso as to disconnect operatingbattery from the relay R6 at that terminal. The relay R5; howevenis slowreleasing. ATherefore, the carrier apparatus-in the transmittinglcircuit 36 at 'the carrier terminal CT -willbel maintained energized 4tomaintain transrnis-sion' of the carrier f1 (165 kilocycles) vover theline: I rfor a long enough time to permit the relay R2 at thesubscribers station to be operated and locked so asto revert thetransmitting frequency ofthe subscribersstationfrom f2 (195 kilocycles)-to ,f3 "(1851: kilocycles) dueE rto thesubstitution of '15 the crystalY2'for the crystal YI in the frequencydetermining circuit of the carrieroscillator at that station. This, however, will not result in theoperation of the relay R4 at the carrier terminal CT by the receivedreverting frequency f3, because :the relay R3 at that terminal isalready released and its contacts provide a break in the energizingcircuit for relay RII from the demodulator 56. When the relay R3released, it also opened the line loop 4I to the central office COgiving a signal to the operator at that oillce. This condition will bemaintained until the relay R5 at the carrier terminal completelyreleases to deenergize the carrier transmitter at the terminal andremove the outgoing carrier f1 (165 kilocycles) from the line I thusreleasing the relay R2 at the carrier subscribers station. The releaseof the relay R2 at the subscribers. station changes the carrierfrequency transmitted from that station from fs (185 kilocycles) to f2(195 kilocycles) and this frequency is transmitted provided theswitchhook is again operated. This causes the relay R3 at the carrierterminal CT to be again operated to close the loop to the central oiceCO and thus signal the operator. This operation should tend to make allflashes from the subscriber slow in character.

Reverting call-between carrier subscribers in the same channel on thepower Zine To make an inter-party line or reverting call, say to asubscriber at station B, the subscriber at station A will remove hishandset i3 from the hook switch 29 causing the transmission apparatus 2Dand 2I in his transmitting circuit I5 to be energized so as to transmithis normal transmitting carrier f2 (kilocycles) to the line I Thesubscriber will then either dial or pass the listed number to theoperator at the central office CO by talking in his transmitter tomodulate his transmitted carrier of frequency ,f2 (195 kilocycles) inthe manner previously described in connection with a central ofce calloriginated by the subscriber.

The calling subscriber at station A will then hang up, and the operatoror machine switching equipment at the central office CO will apply codedEil-cycle ringing current to the voice frequency line `6I which, in themanner previously described for a central oce call, will cause thecarrier transmitter at the carrier terminal CT to be energized to sendcarrier of the frequency f1 (165 kilocycles) out over the line modulatedwith the coded -cycle ringing current from the central oce. The carrierf1 modulated with 20- cycle ringing current will be received anddetected in the receiving circuits of all of the subscribers sets on theline I causing the operation of the control relay RI at all of thesestations to produce operation of the ringer 34 in accordance with thecode as previously described.

When the called subscriber at station B answers his coded ring byremoving his handset I8 from the switchhook 29, the carrier apparatus inhis transmitting circuit I5 is energized so as to send out his normalcarrier frequency f2 (195 kilocycles) to the line I and over that lineto the carrier terminal CT where it will cause the operation of therelay R3 associated with the demod-ulator 52 in the receiving circuit3l' which selects the normal carrier frequency. The closure of the lineloop 4I to the central ofce CO at the carrier terminal CT by operationof the relay R3 operates to trip the ringing from that office. Theoperation of the relay R3 at the carrier terminal also causes theoperation of the relay R6 which energizes the carrier apparatus in thetransmitting circuit 36 at that terminal so that the carrier frequencyfi kilocycles) is sent out through the coupler device I9 over the lineI. During the ringing phase of the call, the' signaling operations areas described above for a central ofce call originated through thecentral omce CO.

When the calling subscriber at station A notes that the ringing hasstopped and that the called party has answered, indicated by thelighting up of the formers busy signal lamp 35, the calling subscriberat station A again removes his handset I8 from the hook switch 29causing the oscillator-modulator 29 and high frequency amplifier 2l inthe transmitting circuit I5 at his station to be energized from battery3I through operated switching contacts on the hook switch. At this time,the relay R2 at the calling subscribers station is operated and lockedfrom battery 3i through its operated right-hand front contact due to theprevious operation of the relay RI in response to the rectied continuouscarrier of the frequency fr (165 kilocycles) received over the line Ifrom the carrier terminal CT. The operated relay R2 through itsright-hand back contacts causes the 195-kilocycle crystal YI to beremoved from the frequency determining circuit of themodulator-oscillator 29 and the 135- kilocycle crystal Y2 to besubstituted therefor in that circuit. Thus, the energized transmittingcircuit I5 at the subscribers station A will send out continuously acarrier wave of the reverting frequency f3 (185 kilocycles) while thehandset I8 remains olf the hook. This carrier wave will be amplied inthe high frequency amplier 2| and will pass through the transmittingband lter 22, plug-in pad 23 and coupler device 9 to the line I overwhich it Will be transmitted to the carrier terminal CT.

At the carrier terminal CT, the received carrier Wave of the frequencyf3 (185 kilocycles) will pass through the coupler device I0 and will beselected by the receiving band filter 53 in the receiving circuit 38.The selected carrier will be amplified by the amplier 54 and berectified in the demodulator 56. The rectified output of the demodulator56 will cause the operation of the associated relay R4 which can beoperated at this time because the relay R3 which controls the energizingcircuit of the latter relay has been previously operated in response tothe rectified carrier wave of the frequency fz (195 kilocycles)transmitted from the called subscribers station B, the two relays R3 andRA providing joint holding paths for the called and calling subscribers,respectively. Each relay R3 and Ril by operating provides a separateclosure of the voice frequency loop 4I to the central oflice CO andholds relay R6 operated from batteries 58 and 59, respectively. Theoperation of the relay R4 also closes the connection from the output ofthe demodulator 56 in the receiving circuit 38 to the hybrid coil 39,and connects the auxiliary balancing network 44 suitable for use on areverting call to the network terminals of hybrid coil 39 in place ofthe normal balancing network 4D used on central office calls. Thus adirect talking connection between the calling subscriber A and thecalled subscriber B is provided through the carrier terminal CT.

The calling subscriber at station A by talking into his transmitter I1modulates the carrier or" the reverting frequency fa kilocycles) in hisoscillator-modulator 20, and the voice-modulated 17 carrier wave of thisfrequency in amplified form is transmitted through the transmittingcircuit l and the coupling device 9 to the power line I and over thatline to the demodulator 56 in the receiving circuit 38 at the carrierterminal CT in the manner previously described for the unmodulatedcarrier wave of that frequency. The demodulated voice signals ofsubscriber A in the output of the demodulator 56 will be transmittedthrough the partially unbalanced hybrid coil 39 to the transmittingcircuit 36 at the carrier terminal CT, in which it will be remodulatedon the carrier frequency f1 (165 kilocycles) in the modulator-oscillator45 held energized by operated relay R6. The voice-modulated carrier waveof the frequency f1 will then be amplified in the amplifier 41 and willpass out through the transmitting band filter 48 and the coupler deviceI6 to the power line I over which it will be transmitted to subscriberstation B. At the subscriber station B, the voice modulated carrier offrequency f1 will be diverted through the coupier device 9 and equalizerpad 24 into the receiving circuit I6 of the station in which it will beselected by the receiving band filter am- 1 plii'led by the amplifier 26and passed to the demodulator 28 in which As voice signals will bedemodulated and will be heard by subscriber B in his receiver I9.

Similarly, the speech signals from the called subscriber B at hisstation modulates a carrier wave of the normal frequency f2 (195kilocycles) in the transmitting oscillator-modulator 26, and themodulated carrier wave will be amplified by high frequency amplifier 2|and transmitted through transmitting band lter 22, plug-in pad 23 andcoupler device 9 to the power line I over which it will be transmittedto the carrier terminal CT. At the carrier terminal CT, the carrier waveof frequency fz (195 kilocycles) modulated with subscriber Bs voicesignals, will be diverted through the coupler device I0 into thereceiving circuit 31 in which it will be selected by the band filter 49,amplified by the amplifier 50 and demodulated to voice frequencies inthe demodulator 52. Bs voice signals will pass from the output ofdemodulator 52 through the partially unbalanced hybrid coil 39 to thetransmitting circuit 36 of the carrier terminal in which they will beremodulated on the transmitted carrier wave of the frequency f1 (165kilocycles) in the oscillator-modulator 45, and the voicemodulatedcarrier will be passed out in the amplied form to the power line I overwhich it will be transmitted to the calling station A. At thesubscribers station A, the carrier wave of the frequency f1 modulatedwith Bs voice signals will be diverted through the coupler device 9 andequalizer pad 24' into the receiving circuit I6 in which it will beselected by filter 25, amplified by amplifier 26 and demodulated bydemodulator 28, and the demodulated voice signals of B will be heard bysubscriber A in his receiver I9.

When the call between subscribers stations A and B is completed, bothsubscribers will hang up their handsets I8 which will automaticallydeenergize the modulator-oscillator 26 and high frequency amplifier 2|in each transmitting circuit I5, stopping the transmission from thestation of the carrier oscillations of the frequencies f3 and f2,respectively. The cessation in the supply of the carrier waves of thefrequencies f2 and f3 at the carrier terminal CT will result in therelease of the switching relays R3, R4 and R6 at that terminal,returning the circuit at that 18 terminal to the normal condition shownwith the carrier transmitter deenergized and the nor'- mal balancingnetwork 46 associated with the hybrid coil 39. The deenergization of thetransmitting circuit at the carrier terminal cutting off thetransmission of the carrier frequency f1 (165 kilocycles) will result inthe release of the switching relays RI at subscribers stations A and B.The consequent release of relay R2 at the calling subscribers station Awill extinguish the busy signal lamp 35 and will cause the crystal Y2 tobe disconnected from the frequency-determining circuit of themodulator-oscillator 20 and the crystal YI to be substituted therefor soas to revert the modulator-oscillator 20 at that station to its normaltuning (f2-195 kilocycles).

If, during a reverting or central office call, a third carriersubscriber removes his handset I8 from the swtchhook, the latterstransmitter will send out carrier oscillations of the revertingfrequency f3 (185 kilocycles) over the line I because of the presence ofcarrier oscillations f1 on that line, holding the relay R2 at the thirdsubscribers' station locked operated. This will cause beat notes anddistortion of the speech between the other parties-engaged in the call,since the transmitted carrier frequency of the third subscriber will inmost cases be slightly different from that of thesubscriber previouslyusing the carrier frequency-fs. In spite of this distortioninterference, transmission will probably be suiiiciently good to providea means for explaining the emergency condition which would require thistype of break-in by a certain subscriber. Such a breakin will notrelease the circuit, and transmission will return to normal as soon asthe third party hangs up. The operator can always break in on either acentral office or a reverting call if requiredby an emergency.

The invention has been described as applied to only one carrier channelon the power line I, but it is apparent that a number of differentchannels may be operated on the same line by the assignment of threedierent carrier frequencies for each channel, in which case separatecarrier terminals associated with the same or different centralswitching offices would be employed for each individual channel. Theapparatus and circuit arrangements at the subscribers stations andcarrier terminals associated with the additional carrier channels may beidentical with those in the system as described above and illustrated inthe drawings, except for the changes required in the carrier oscillatorsand band lters due to the use of different carrier frequencies for theseveral channels. Other changes in the circuits illustrated anddescribed which are within the spirit and the scope of the inventionwill occur to persons skilled in the art.

What is claimed is:

1. In a carrier communication system, a transmission medium and stationscoupled to said medium, including a carrier terminal station and aplurality of carrier subscriber stations, said carrier terminal stationincluding a transmitter for transmitting communication signals to saidmedium on a carrier wave of one frequency and two receivers forrespectively detecting communication signals received over said mediumon a carrier wave of a second frequency and a carrier wave of a thirdfrequency, each of said carrier subscriber stations including atransmitter normally conditioned for transmitting to said medium on acarrier wave of said second frequency only,V switching meansresponsiveto applied carrier waves of said one frequency received over said mediumfrom said carrier terminal station for conditioning the transmitter atthe local subscriber station for transmitting to said medium on acarrier wave of said third frequency only and other switching means forpreventing operation of the first switching means if said localsubscriber station is transmitting to said medium on a carrier wave ofsaid second frequency at the time of arrival of a carrier wave of saidone frequency thereat over said medium.

2. In a carrier communication system, a transmission medium and aplurality of stations coupled to said medium, including a carrierterminal station and a plurality of carrier subscriber stations, saidcarrier terminal station including a transmitter for transmittingcommunication signals on carrier waves of one frequency to said mediumfor transmission thereover to all of said carrier subscriber stationsand two receivers for respectively detecting communication signalsreceived over said medium on a carrier wave of a second and a thirdfrequency, each of said carrier subscriber stations including atransmitter normally conditioned for operation under control of thesubscriber at the station to transmit communication signals to saidmedium on a carrier wave of said second frequency only, switching meansresponsive to carrier waves of said one frequency received over saidmedium to condition the transmitter at the local subscriber station fortransmitting to said medium on a carrier wave of said third frequencyonly anda second switching means operating to disable the firstswitching means only if the transmitter at the station is made operativeto transmit carrier waves of said second carrier frequency to saidmedium prior to arrival of the carrier waves of said one frequency atthe subscriber station.

3. In combination in a carrier communication system, a transmissionmedium and stations including a carrier terminal station and a pluralityof carrier subscriber stations, coupled to said medium, said carrierterminal station including a transmitter for sending communicationsignals on a carrier wave of one frequency out over said medium to saidsubscriber stations, and two receivers for detecting communication sig-lnals received over said medium from said subscriber stations on acarrier wave of a second frequency and a third frequency, respectively,each of said subscriber stations including a receiver for detectingcommunication signals re` ceived over said medium on a carrier wave ofsaid one frequency and a transmitter for transmittingcommunicationsignals over said medium to said carrier terminal either on a carrierwave of said second frequency or a carrier wave of said third frequency,the latter transmitter being normally conditioned for operation undercontrol of the local subscriber to transmit on a carrier wave of saidsecond frequency only, relay switching means responsive to the waveenergy in the output of the local receiver when a carrier wave of saidone frequency is received at the station over said medium forconditioning the transmitter at the station for operation under controlof the local subscriber to transmit over said medium on a carrier waveof said third frequency only and other switching means controlled by thelocal subscriber when he seizes control of the local transmitter, onlyin the absence of operation of the first switching means by carrier waveenergy of said one frequency previously received at the -station oversaid medium, for preventing operation of said first switching means lnresponse to carrier wave energy of said one frequency subsequentlyreceived over said medium.

4. In a carrier communication system, a transmission line, stationscoupled to said line including a different group of carrier subscriberstations and an individual carrier terminal station for each carrierchannel thereon and a central switching ofce associated with the carrierterminal stations, the individual carrier terminal station for onecarrier channel including a normally de-energized signal transmitterwhich, when energized, is operative to send out communication signals ona carrier wave of one frequency over said line to said subscriberstations and two receivers for detecting communication signals receivedover said line on a carrier wave of a second frequency or on a carrierwave of a third frequency, respectively, each of the subscriber stationsfor said one carrier channel including a receiver for detectingcommunication signals received over said line on a carrier wave of saidone frequency, a transmitter normally adapted under control of thesubscriber at the station for sending out communication signals oversaid line on a carrier wave of said second frequency only, relayswitching means responsive to the wave output of the local receiver whencarrier wave energy of said one frequency is received at the stationover said line to condition the transmitter at the station for operationunder control of the subscriber thereat to send out communicationsignals over said line on a carrier wave of said third frequency andmeans also under control of the subscriber at the station forveifectively disabling said switching means if he causes the transmitterat the station to be rendered operative prior to the arrival at thestation over said line of carrier waves of said one frequency.

'5. The system of claim 4 in which said carrier terminal station forsaid one carrier channel also includes means responsive to ringingsignals received from said central oftice or carrier waves of saidsecond frequency received over said line from one of said subscriberstations for said one channel for energizing the local transmitter meansresponsive to carrier waves of said third frequency received over saidline for holding the transmitter in the energized condition.

6. In a carrier communication system, a main transmission line, aplurality of stations coupled to said line including an individualcarrier terminal station and a different group of carrier subscriberstations associated with each carrier channel on said line, a centralswitching oice for said stations on said line connected lthereto throughthe carrier terminal stations, and adapted also for connecting outsidesubscriber communications stations on other lines in communicationrelation with the carrier subscriber stations in each group on said mainline through its individual carrier terminal station, each of thecarrier subscriber stations in the group assigned to one carrier channelincluding a receiver operative to detect communication signals receivedat the station over said main line modulated on a carrier wave of afrequency f1, a transmitter normally conditioned under control of thesubscriber at the station for sending out communication signalsmodulated on a carrier wave of a second frequency f2 over said main lineto the carrier terminal station for that channel on calls between thatstation and said central oiiice or an outside subscriber stationconnected through said office, switching means responsive to acontinuous carrier wave of the frequency f1 received over said main linefrom the latter carrier terminal station only if the transmitter at thesubscriber station at the time is not transmitting to said main line ona carrier Wave of the frequency f2, to condition that transmitter forsending out under control of the local subscriber communication signalsmodulated on a carrier wave of a third frequency over said main line,for a reverting call between the local subscriber station and anothercarrier subscriber station in the same group on the main line initiatedat the local station, the carrier terminal station for said one carrierchannel including a transmitter operative under control of call vsignalsfrom said central oice or in response to a carrier wave of the frequencyf2 received over said main line from one of the carrier subscriberstations associated with said one channel,

to send out a carrier Wave of the frequency f1 over said main line tothe carrier subscriber stations, two receivers for respectivelyselectively receiving the carrier Waves of the frequency fz and faincoming over said main line and for demodulating the communicationsignals therefrom and means to supply the demodulated signals to saidcentral oice and to the transmitter in the carrier terminal station forremodulation on the carrier wave of the frequency f1 sent out therebyover said line to said carrier subscriber stations.

7. The system of claim 6, in which said carrier terminal station forsaid one carrier channel includes means for causing the signal output ofthe receiver thereof for selectively receiving carrier waves of thefrequency f3 to be connected to said central office and to thetransmitter at that station only when carrier waves of both frequenciesfz and f3 are being received over said line at said carrier terminalstation.

8. The system of claim 6, in which said central ofce is connected to thetransmitter and two receivers at the carrier terminal station for saidone carrier channel through a voice frequency line, a hybrid coil and anassociated normal line balancing network suitable for providing adequatebalance of any of said other lines which may be connected incommunication relation through the central oce with the carriersubscriber stations on said main line, and said carrier terminal stationfor said one channel includes switching means responsive to a carrierwave of said third frequency received thereat over said main line forsubstituting an auxiliary balancing network for said normal network,said auxiliary balancing network being suitable for providing a desiredamount of unbalance of said hybrid coil so as to enable a suiicientamount of the demodulated signal energy output of said receiver forselectively receiving carrier waves of said frequency f3 to betransmitted through said hybrid coil to the transmitter at the carrierterminal station on a reverting call.

ROBERT C. EDSON. JOHN W. EMLING.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,705,891 Arnold Mar. 19, 19291,773,613 Clark Aug. 19, 1930 2,289,048 Sandalls July 7, 1942 2,292,222Haigis Aug. 4, 1942 2,430,471 Lang Nov. 11, 1947 OTHER REFERENCESTechnical Manual 11-600 pages 8; 67-68 March 3, 1943.

